In a contemporary detection system, a viewed target or scene forms a single image upon a focal-plane detector array including a large number of discrete detector elements that are highly responsive to electromagnetic energy within a pre-selected wavelength range. The electrical outputs of the detector elements are communicatively linked to sophisticated signal processing circuitry. By rapidly analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and monitor sources of electromagnetic radiation that appear within a scene or field of view.
When it is desired to view a scene over different portions (wavelength ranges) of the electromagnetic spectrum, the scene is filtered through one or more optical filtering elements. In a traditional system, mechanically movable filters are interposed into an optical path defined by a focusing element and a corresponding section of the detector array onto which that focusing element projects an image of the scene. Generally, such filtering elements are selectively situated intermediate the focusing element and the detector array.
Practitioners in the field of spectral imaging have recognized shortcomings of systems that rely on the selective mechanical interposition of filters within an optical path in order to image a scene over different wavelength ranges. Among the disadvantages associated with such systems are the facts that they are inherently expensive, heavy, large and fragile. More specifically, the use of mechanisms to effect movement of the filters adds costs and weight to the detection system. More significantly, such mechanisms are mechanically complex and require a high degree of precision to obtain the desired results. Thus, the reliability and durability of moveable filters, and their drive mechanisms, are of particular concern. This is especially true in space-based applications wherein it is extremely difficult or impossible to conduct “field” repair of such systems. Mechanical movement of the filters also introduces an observation dead time associated with (i) the generation of control signals to initiate the filter change, (ii) settle-down times that depend on the inertial characteristics of the mechanical components, (iii) and slow speeds that may be necessary in order to preserve optical alignment, avoid setting up vibration, and prevent damage to fragile optics. In some military systems requiring extremely rapid response times, any time loss associated with filter switching may be highly undesirable or even unacceptable. Moreover, and quite significantly, moveable filters (e.g., filter wheels) provide spectral data that is necessarily sequential in nature. More specifically, a scene is viewed through a first filter and data representative of the scene is registered at the detector array and stored in computer memory. Subsequently, a second filter is moved into position to filter the scene over another wavelength range, and the procedure is repeated over as many filtered wavelength ranges as the particular application calls for. Much more desirable is the acquisition of all spectral data through all filter elements simultaneously. This is particularly important when viewing rapidly changing events such as missile launches, muzzle flashes, or other ephemeral events.
In recognition of the aforementioned considerations, multi-image detector assemblies have been developed. Such an assembly does not require the use of moveable filters or other optical components in order to sense different portions of the electromagnetic spectrum or otherwise modify the incoming source signal. Representative of such an assembly is that disclosed in U.S. Pat. No. 5,479,015 issued in the names of Rudman et al. on Dec. 26, 1995 (hereinafter, the '015 patent). The '015 patent is drawn to a multi-image detector assembly including an array of detector elements (e.g., a focal plane array) wherein the detector array includes a plurality of imaging-registering sections. Corresponding to each image-registering section is a focusing member that focuses an image of a scene upon the image-registering array section. The plural images focused upon the various array sections are, at least in their spatial aspects, substantially identical.
Each focusing member defines, in combination with its corresponding array section, an optical path. Disposed within, and dedicated to, each optical path is an optical element that modifies the image transmitted along that optical path. The “optical elements” are optical filters that facilitate registration of various images of a single scene simultaneously over disparate wavelength regions within the electromagnetic spectrum.
While the assembly of the '015 patent certainly alleviates the aforementioned shortcomings of systems employing sequential, mechanically-driven filter substitution, systems relying on either sequential or simultaneous image acquisition include certain undesirable or limiting characteristics. For instance, in the traditional sequential image-acquisition system represented by the inclusion of a mechanical filter wheel, for example, a “full-sized” image is registered at the detector array, and data representative of that image is stored in computer memory, for each filtered wavelength range. The data representative of each full-sized image is then algorithmically analyzed in order to render determinations as to whether at least one event of interest appears in each image. This sequential analysis is undertaken continuously over full-sized images in each predetermined wavelength range whether an event of interest is present or not in the monitored field of view. Similarly, while the “full-sized” images associated with a simultaneous image-acquisition system such as that of the '015 patent may be smaller than the full-sized images associated with a sequential image-acquisition system because the image associated with each filtered wavelength range of interest is focused upon a dedicated fraction of the overall detector array, a system such as that of the '015 patent still continuously and simultaneously acquires what, relative to it, are full-sized images over each filtered wavelength range and analyzes these in search of interesting events. In other words, it is a common objective of the previous systems to continuously utilize as much of the available area of the detector array as possible in order to maximize the resolution of each acquired image. In either case, maximized use of the detector-array surface area produces a continuous series of computationally-intensive data frames requiring storage and analysis, even when there are no events of interest present in various frames over various wavelength ranges. Another shortcoming of contemporary imaging systems, whether they capture images sequentially or simultaneously, is that they include a single filter and lens pair corresponding to each wavelength range over which they are designed to capture images. Accordingly, for example, in a typical multi-image detector assembly such as that disclosed in the '015 patent, if one of the focusing members, or its corresponding optical filter incurs mechanical damage, the system will no longer be capable of cleanly imaging a scene over the wavelength range to which the damaged focusing member, or its corresponding optical filter, corresponds unless and until repairs can be made. Under certain conditions, especially those relating to a military operation, a damaged lens of optical filter could deny to personnel or their weapons system critical data regarding a potential threat or target.
Accordingly, there exists a need for a multi-image detector assembly that, in alternative embodiments, at least one of (i) provides redundant images relative to each wavelength range of a selected set of wavelength ranges over which a detection system is designed to register images of a scene, and (ii) facilitates monitoring of a field of view through the simultaneous acquisition of correlated images over multiple, predetermined ranges of the electromagnetic spectrum by utilizing some minimized portion of a detector array while in a “monitoring” mode and that selectively utilizes a larger area of the detector array when circumstances justify the consequent consumption of increased computer memory and analytical resources.